Field of the Invention
The present invention relates to a steel pipe obtained through quenching and tempering treatment and excellent in fatigue characteristics, especially to such a quenched/tempered steel pipe for machine structural members that has been designed to have a high strength by increasing the hardness thereof and to have a high fatigue life by precipitating fine carbides therein, and relates to a method for producing it.
Background Art
In various machine structures such as typically automobiles, often used are quenched/tempered “steel pipes” for members that are required to have a high strength and good fatigue characteristics.
In general, for improving the fatigue characteristics of steel materials, surface hardening or smoothing is said to be effective.
Patent Reference 1 discloses a technique of improving fatigue characteristics by surface hardening through nitriding treatment. Patent Reference 2 discloses a technique of improving the fatigue characteristics of steel pipes by grinding the inner surface of a steel pipe for smoothing and for decarburized layer removal.
[Patent Reference 1] JP-A 6-264177
[Patent Reference 2] JP-A 7-215038
Nowadays, various members of machine structures are increasingly required to be downsized and lightweight. High-strength members composed of steel pipes are no exceptions.
For making steel pipe members lightweight, it is most effective to reduce the pipe wall thickness thereof. However, thin-walled structures are disadvantageous in point of the strength and the fatigue life thereof. In particular, steel pipes are often worked to have a desired shape by bending or the like, but the outside wall of the bent part has a reduced thickness and is in a severe state in point of the durability. Accordingly, for satisfying the requirement for wall thickness reduction, it is desired to improve the level of the characteristics of steel pipes themselves, or that is, to elevate the fatigue life of steel pipes to a further higher level with maintaining the high strength thereof.
It is not always easy to reduce the pipe wall thickness of high-strength steel pipes with maintaining the durability thereof to that effect. As a means for solving the problem, for example, a method may be taken into consideration of improving the strength/fatigue characteristic level of steel materials themselves by addition of specific elements thereto. However, for many machine structures, such a method that may bring about material cost increase is unacceptable. The method of surface nitriding as in Patent Reference 1, and the method of inner surface grinding as in Patent Reference 2 may be effective for improving fatigue characteristics, but both involve increase in process steps; and the current steel pipe production process could not be directly applied thereto. The method of Patent Reference 2 has another problem of process yield reduction.
Accordingly, it is not easy to increase the level of the fatigue life of high-strength steel pipes with maintaining the high strength thereof according to an inexpensive method, and at present, such a method is not as yet established.
In consideration of the current situation as above, the present invention is to provide a steel pipe that is designed to have a further increased high strength and a further improved fatigue life, especially to such a steel pipe suitable for wall thickness reduction in hollow stabilizers for automobiles.